Polymer recycling

Abstract

A method for recycling polyethylene terephthalate (PET) to produce bis(2-hydroxyethyl) terephthalate (BHET) comprises the steps of (a) depolymerising PET in the presence of ethylene glycol and a catalyst system in a series of preferably two depolymerisation reactors to form a depolymerised mixture comprising BHET; (b) crystallising a precipitate comprising BHET from the depolymerised mixture; (c) dissolving the precipitate in a protic solvent, preferably water, but also optionally methanol, to form a solution comprising BHET; (d) removing impurities from the solution to form a purified solution comprising BHET; and (e) crystallising a purified product comprising BHET from the purified solution. Suitable apparatus for such a method, and the use of urea in a catalyst system therefor, are also provided.

The invention claimed is: 
     
       1. A method for recycling polyethylene terephthalate (PET), said method comprising:
 (a) depolymerizing PET in the presence of ethylene glycol and a catalyst system in a series of depolymerization reactors to form a depolymerized mixture comprising bis(2-hydroxyethyl)terephthalate (BHET); 
 (b) crystallizing a precipitate comprising BHET from the depolymerized mixture; 
 (c) dissolving the precipitate in a protic solvent to form a solution comprising BHET; 
 (d) removing impurities from the solution to form a purified solution comprising BHET; and 
 (e) crystallizing a purified product comprising BHET from the purified solution. 
 
     
     
       2. The method of  claim 1 , wherein the PET is waste PET, wherein the PET is used in the form of particles where:
 at least 80% by weight of the particles pass through a mesh having openings with a diameter of 20 mm; 
 100% by weight of the particles pass through a mesh having openings with a diameter of 25 mm; and/or 
 up to 1% by weight of the particles pass through a mesh having openings with a diameter of 0.1 mm. 
 
     
     
       3. The method of  claim 1 , wherein the PET has a b[h] value of greater than 5. 
     
     
       4. The method of  claim 1 , wherein each of the depolymerization reactors used in step (a) is operated:
 at a temperature of from 150 to 230° C.; 
 at atmospheric pressure; 
 for a period of from 20 minutes to 4 hours; and/or 
 with stirring. 
 
     
     
       5. The method of  claim 1 , wherein ethylene glycol is used in step (a) in amount of from 2 to 6 times the amount of PET by weight. 
     
     
       6. The method of  claim 1 , wherein the catalyst system comprises a transition metal catalyst. 
     
     
       7. The method of  claim 1 , wherein the catalyst system comprises a carrier. 
     
     
       8. The method of  claim 1 , wherein the catalyst system is used in step (a) in an amount of from 0.001 to 0.5 times the amount of PET by weight. 
     
     
       9. The method of  claim 1 , wherein step (b) is carried out using cooling crystallization:
 by reducing the temperature of the depolymerized mixture to a temperature of from 5 to 50 at atmospheric pressure; 
 for a period of from 10 to 60 minutes; and/or 
 under stirring. 
 
     
     
       10. The method of  claim 1 , wherein the protic solvent comprises one or more of water, methanol, ethanol, iso-propanol, and n-butanol. 
     
     
       11. The method of  claim 10 , wherein step (c) is carried out:
 at a temperature of from 40 to 95° C.; 
 at atmospheric pressure; 
 for a period of from 1 to 60 minutes; and/or 
 under stirring. 
 
     
     
       12. The method of  claim 10 , wherein water is used in step (c) in an amount of from 0.1 to 2.5 times the amount of PET used in step (a) by weight. 
     
     
       13. The method of  claim 1 , wherein step (d) comprises decolorizing the solution by contacting the solution with carbon, an ion exchange resin, and/or a clay. 
     
     
       14. The method of  claim 1 , wherein step (e) is carried out using cooling crystallization:
 by reducing the temperature of the purified solution to a temperature of from 0 to 55° C.; 
 at atmospheric pressure; 
 for a period of from 10 to 60 minutes; and/or 
 under stirring. 
 
     
     
       15. The method of  claim 1 , wherein the depolymerized mixture is passed through a filter between steps (a) and (b) to remove insoluble components. 
     
     
       16. The method of  claim 1 , wherein the precipitate comprising BHET is isolated between steps (b) and (c), and wherein the filtrate is recycled to the first depolymerization reactor in step (a). 
     
     
       17. The method of  claim 1 , wherein the protic solvent is methanol, and the method comprises isolating the purified product comprising BHET after step (e), wherein the method comprises processing the filtrate to recover methanol and ethylene glycol, and recycling methanol to step (c) and/or ethylene glycol to a depolymerization reactor in step (a). 
     
     
       18. The method of  claim 1 , wherein the method further comprises:
 (f) drying the purified product comprising BHET. 
 
     
     
       19. The method of  claim 1 , wherein the purified product comprising BHET has a b/[h] value of up to 2; and/or
 wherein the purified product comprises: 
 BHET in an amount of at least 95%; and 
 dimers and trimers of BHET in an amount of at least 0.01% by weight. 
 
     
     
       20. An apparatus for recycling polyethylene terephthalate (PET), said apparatus comprising:
 (a) a series of depolymerization reactors which are adapted for depolymerizing PET to form a depolymerized mixture comprising bis(2-hydroxyethyl) terephthalate (BHET), wherein the series of depolymerization reactors is adapted to receive PET, ethylene glycol and a catalyst system; 
 (b) a crystallization unit downstream of the polymerization reactors adapted for crystallizing a precipitate comprising BHET from the depolymerized mixture; 
 (c) a vessel for receiving the precipitate and which is adapted for dissolving the precipitate in a protic solvent to form a solution comprising BHET; 
 (d) an impurity removal unit for receiving the solution comprising BHET and which removes impurities from the solution to form a purified solution; and 
 (e) a further crystallization unit downstream of the purity removal unit suitable for crystallizing a purified product comprising BHET from the purified solution.